Low side hermetic refrigeration compressors are those in which most, if not all, of the interior of the shell is at suction pressure. Normally, some, or all, of the suction flow is used to cool the motor which is provided with a thermal protector. The thermal protector causes the motor, and thereby the compressor, to stop when the motor overheats.
In U.S. Pat. No. 5,141,407 it is recognized that hot, discharge gas could cause the thermal protector to react and stop the compressor. To cause the thermal protector to react, a discharge to suction/shell interior bypass is controlled by a thermally responsive valve which senses and reacts to the temperature of the discharge gas. As a result, the compressor can be stopped responsive to conditions resulting in an excessive discharge temperature. These conditions include the loss of working fluid charge, a blocked condenser fan in a refrigeration system and a low pressure condition or a blocked suction condition. Thus, the thermal protection disclosed in U.S. Pat. No. 5,141,407 is basically that of "general heat" where the heat is generated all around the scroll as friction heat caused by lack of lubrication, the thermodynamic heat of the compressed gas, high motor temperature and/or high ambient temperature. The basic presumption of this approach is, however, that the discharge gas temperature always follows closely the actual failure indication which is not always true.
In addition to general heat there can be "local heat" which is heat generated in a certain area. The source of local heat is usually localized high friction caused by a concentrated load. With local heat, the amount of total heat may not be sufficient to significantly influence the temperature of the discharge gas such as under the high mass flow conditions associated with a blocked condenser fan. Thus, a gas temperature sensing device may not detect an incipient failure caused by local friction.
Scroll compressors are unusual in that there is a continuous progression of the compression process from the outermost suction region to the inner discharge region and in that relative movements between contacting points on the two scrolls is limited to a circle, the orbit, which is typically 0.5 inches or less. As a result, there is a thermal gradient from the outer periphery to the center of the scrolls and contact between the members is localized. The wraps of a scroll compressor exhibit a differential thermal growth reflecting the thermal gradient, with the inner portion of the wraps having the greatest thermal growth. A "worn in" scroll wrap will, typically, be dished concavely at ambient temperatures and planar at operating temperatures. During abusive conditions such as loss of working fluid charge, the compressor may operate at high pressure ratios which can lead to high discharge temperatures. Due to thermodynamic heat, the resultant thermal gradient causes the inner portion of the scrolls to expand beyond the "normal" planar state and results in convex dishing. This will cause the axial thrust load to be concentrated on a very small area near the center of the scroll wrap. The failure mechanism for a scroll compressor under these conditions could be excessive wear of the scroll surface and/or galling near the center. Galling is a continuous weld-tear between the wrap tip and floor of coacting scroll members. The major factors that contribute to failure are (1) heat generated in the compressor which causes breakdown of oil, reducing lubrication and increasing friction and friction heat between the scrolls, and (2) high net axial thrust force or concentrated thrust loading between the scrolls which can increase friction and create more friction heat.
Loss of working fluid charge creates significant local and general heat. As charge is lost from the system, the discharge to suction gas pressure ratio increases. As the pressure ratio increases, the temperature difference between suction and discharge increases and results in dishing of the scroll members which eventually creates a high spot. The high spot takes all the load (normal force) and causes high local friction and resultant local heat. Additionally, because the lubrication media is oil entrained in the refrigerant, the reduction in mass flow reduces the available lubrication for the scrolls, increasing friction and its resultant general heat. The normal thermodynamic heating of the gas will also provide general heat.